JP6500389B2 - Method of manufacturing hot rolled steel sheet - Google Patents

Description

The present invention relates to a method of manufacturing a hot rolled steel plate for producing hot stamping steel plate.

  BACKGROUND ART In recent years, hot stamps are being widely used as a technique for manufacturing a high-strength component of 1000 to 2000 MPa for use in automobiles and the like with high dimensional accuracy. Hot stamping is a technology for press forming in a state where a steel sheet is heated to an austenite region and simultaneously performing forming and hardening to improve the dimensional accuracy of the formed product and to increase the strength.

  Generally, a steel plate for hot stamping contains a relatively large amount of C to secure the strength of a molded product, and requires Mn and B (austenite stabilizing element) to secure hardenability during mold cooling. Although this is contained, this point acts disadvantageously in the production of the steel sheet.

  For example, in a coil obtained by winding a hot-rolled steel sheet containing a required amount of Mn and B to secure hardenability, the cooling rate is different at the portion of the coil and the microstructure becomes uneven, which is caused by this Materials tend to be hard. That is, it is thought that the unevenness of the microstructure in the heat-rolled steel sheet leads to the deterioration of the cold workability.

  In addition, when a large amount of C is contained to secure the strength of a formed product, the hardness of the hot-rolled steel sheet excessively increases, and the next cold rolling becomes difficult.

  Although tempering by batch annealing can be considered as a means to eliminate the unevenness of the microstructure and secure the cold workability, it usually takes 3 to 4 days, so it is not only undesirable from the viewpoint of productivity, but also carbides There is a problem that carbides tend to be unmelted at the time of heating at the time of carrying out the hot stamping and the strength after quenching can not be sufficiently obtained because of agglomeration.

  Furthermore, although a method by continuous annealing can be considered from the viewpoint of productivity, it is difficult to sufficiently soften the steel sheet and uniform the microstructure because the annealing time is short.

  If the heat-rolled steel plate or the cold-rolled steel plate is not sufficiently softened and the microstructure is not uniform, the dimensional accuracy of the hot stamped molded product is lowered and the hardness and the strength are varied. Heretofore, there have been proposed some techniques for improving dimensional accuracy of a hot stamped molded product and eliminating variations in hardness and strength (see, for example, Patent Documents 1 to 6).

  For example, in Patent Document 6, in a method of producing a steel plate for hot pressing by hot rolling, cold rolling, or continuous annealing a steel plate having a predetermined component composition, the steel plate is (Ac 3-10 ° C) in the continuous annealing step Heat to (Ac 3-60 ° C.), then cool at 10 ° C./s or less, hold at 550-660 ° C. for 1 to 10 minutes, and then temper rolling, and in the hot stamping step, non-heated parts are present The method is a method for producing a hot stamped product having a small variation in hardness, characterized in that heating is performed in a state where the maximum heating temperature of the quenched portion at this time is Ac3 or more and hot stamping is performed.

JP 2007-169679 A International Publication No. 2012/053637 International Publication No. 2012/120692 JP, 2012-237048, A JP, 2013-122076, A JP, 2013-151708, A

According to the above-mentioned prior art, the softening of the heat-rolled steel plate and the homogenization of the microstructure facilitate the next cold rolling, thereby improving the dimensional accuracy of the hot stamped molded product and eliminating the variation in hardness and strength. Can be The present invention provides a steel sheet containing relatively large amount of C, and an object thereof is to soften the material by homogenizing the microstructure of the hot rolled steel sheet before cold rolling, hot-rolled steel plate to solve the problem The purpose is to provide a manufacturing method of

  The present inventors diligently studied methods for solving the above problems. As a result, the present inventors perform finish rolling in a temperature range lower than Ar 3 point, and after cooling, if the hot-rolled steel plate is reheated for a short time, the microstructure of the hot-rolled steel plate becomes uniform to make the material It has been found that it can be softened.

  The present invention has been made based on the above findings, and the summary thereof is as follows.

( 1 ) In a continuous hot rolling step , C: 0.07 to 0.50%, Si: 0.10 to 1.00%, Mn: 0.70 to 3.00%, P: 0.% by mass. 010% or less, S: 0.010% or less, Al: 0.001 to 0.100%, N: 0.010% or less, O: 0.010% or less, and 20% or more by volume fraction A method of manufacturing a hot-rolled steel sheet having a microstructure including: a hot-rolled steel sheet finished hot-rolling in a temperature range of 600 ° C. or more at a Ar 3 point and cooled to 300 ° C. or less ° C. was heated to 700 ° C. or less above, in the method of manufacturing a hot-rolled steel sheet you characterized by winding it.

( 2 ) In a continuous hot rolling step , C: 0.07 to 0.50%, Si: 0.10 to 1.00%, Mn: 0.70 to 3.00%, P: 0.% by mass. 010% or less, S: 0.010% or less, Al: 0.001 to 0.100%, N: 0.010% or less, O: 0.010% or less, and 20% or more by volume fraction A method of manufacturing a hot rolled steel sheet having a microstructure including : cooling the hot rolled steel sheet which has finished hot rolling in a temperature range of 600 ° C. or higher at Ar 3 point or lower to 300 ° C. or lower, and then rewinding it. method for manufacturing a hot-rolled steel sheet shall be the wherein the winding is heated to 400 ° C. or higher 700 ° C. or less Te.

  According to the present invention, since the microstructure of the heat-rolled steel plate is uniformed and the cold workability is improved, it is possible to increase the rolling reduction and manufacture a thin cold-rolled steel plate having a uniform microstructure. As a result, the dimensional accuracy of the hot stamped molded product is improved, and variations in hardness and strength are eliminated.

It is a figure which shows the process of the manufacturing method of the hot rolled sheet steel of this invention. It is a figure which shows another process of the manufacturing method of the hot rolled sheet steel of this invention. (A) shows the process until it winds up a hot rolled steel plate once, (b) shows the process of rewinding and heating the wound hot rolled steel plate, and winding up again.

The method for producing a hot- rolled steel sheet of the present invention (hereinafter sometimes referred to as “the present production method”) is a continuous hot rolling step, in mass%, C: 0.07 to 0.50%, Si: 0. 10 to 1.00%, Mn: 0.70 to 3.00%, P: 0.010% or less, S: 0.010% or less, Al: 0.001 to 0.100%, N: 0.010 %, O: not more than 0.010%, and a method of manufacturing a heat-rolled steel sheet having a microstructure containing not less than 20% by volume fraction of ferrite, and heat in a temperature range of not less than 600 ° C. The hot-rolled steel sheet which has been subjected to inter-rolling is cooled to 300 ° C. or less, heated to 400 ° C. or more and 700 ° C. or less immediately before winding, and wound as it is.

In the production method of the present invention , C: 0.07 to 0.50%, Si: 0.10 to 1.00%, Mn: 0.70 to 3.00% by mass% in the continuous hot rolling step P: 0.010% or less, S: 0.010% or less, Al: 0.001 to 0.100%, N: 0.010% or less, O: 0.010% or less, and the volume fraction A method of manufacturing a hot rolled steel sheet having a microstructure containing 20% or more of ferrite, wherein the hot rolled steel sheet which has finished hot rolling in a temperature range of 600 ° C. or higher below Ar 3 point is once cooled to 300 ° C. or lower Then, the film is unwound and heated at 400 ° C. or more and 700 ° C. or less to wind it up.

  First, the manufacturing method of the present invention will be described.

  FIG. 1 shows the steps of the manufacturing method of the present invention. Usually, after rough rolling, the rough bar S 'is finish-rolled by a finish-rolling machine to form a hot-rolled steel sheet, cooled by a cooling means, and then wound by a winding machine. A heating means is disposed in front of the removing machine, and the heat-rolled steel plate S immediately before being wounded is heated to a required temperature to soften the material in the thickness direction and the longitudinal direction.

  FIG. 2 shows another process of the method of the present invention. FIG. 2 (a) shows a process until the hot-rolled steel plate is once wound up, and FIG. 2 (b) shows a process of rewinding and heating the wound hot-rolled steel plate and again winding it.

  In the manufacturing method of the present invention shown in FIG. 2, the rough bar S ′ which has been subjected to rough rolling is subjected to finish rolling with a finish rolling mill to form a hot rolled steel sheet, cooled by a cooling means, and temporarily wound by a winder. (Refer to FIG. 2 (a).). The hot-rolled steel sheet S which has been wound up and cooled to room temperature is rewound, heated to a required temperature by a heating means, and the heated hot-rolled steel sheet S1 is wound up again by a winding machine as it is.

  The finish rolling is performed at a temperature range of 600 ° C. or more at Ar 3 point or less and finished. By performing finish rolling at a temperature range of Ar 3 point or less to promote ferrite transformation. The material of the heat-rolled steel plate is softened, and it becomes possible to increase the rolling reduction in the following cold rolling.

  The hot-rolled steel sheet after the end of the hot rolling is cooled to 300 ° C. or less by a normal cooling means. When the cooling temperature exceeds 300 ° C., the degree of expression of the material softening effect that is developed in cooperation with the next heat treatment (400 ° C. or more and 700 ° C.) becomes insufficient. The cooling temperature is preferably 270 ° C. or less.

  Next, the hot-rolled steel plate cooled to 300 ° C. or less is heated to 400 ° C. or more and 700 ° C. or less immediately before winding. Since it is necessary to immediately heat the hot-rolled steel sheet immediately after cooling to 400 ° C. or more and 700 ° C. or less, the heating means is preferably a heating means capable of rapid heating. Fast responsive electric heating is preferable, and for example, electric heating or induction heating is preferable. The heating rate is not particularly limited.

  If the heating temperature is less than 400 ° C., the material softening effect by heating is insufficient, so the temperature is set to 400 ° C. or more. Preferably it is 450 degreeC or more. If the heating temperature exceeds 700 ° C., the temperature fluctuation in the coil caused by the winding becomes large, so the temperature is made 700 ° C. or less. Preferably it is 650 degrees C or less. The hot rolled steel sheet heated to 400 ° C. or more and 700 ° C. or less is wound as it is at that temperature.

  In the manufacturing method of the present invention, immediately after the hot-rolled steel plate is cooled to 300 ° C. or less, the cold workability of the hot-rolled steel plate is significantly improved by the heat treatment of heating to 400 ° C. or more and 700 ° C. or less. Infers as follows.

  During finish rolling, while ferrite transformation is promoted, strain introduced by rolling remains, recrystallization is promoted by reheating, and the material of the hot rolled steel sheet is softened, and the cold workability is remarkably improved. Do. In the heat-rolled steel sheet of the present invention, the Vickers hardness Hv is set to 350 or less as the strength capable of sufficiently securing the cold rolling property.

Hot-rolled steel sheet produced by the present invention production process (hereinafter sometimes referred to as "the present invention hot-rolled steel sheet".) Is a hot rolled steel sheet has finished hot rolled at a temperature range of not lower than 600 ° C. or less Ar3 point, 300 It is characterized in that it contains recovered or recrystallized ferrite as a microstructure formed on the heat-rolled steel sheet by heating to 400 ° C. or more and 700 ° C. or less immediately before winding after cooling to ≦ C or less.

  The heat-rolled steel plate of the present invention is preferably a hot-rolled steel plate which has finished hot-rolling in a (γ + α) two-phase region in a temperature range of Ar3 point to 600 ° C. or higher. By hot rolling in the (γ + α) two-phase region, it is possible to reliably obtain a ferrite-based microstructure of 20% or more by volume fraction.

  The heat treatment effect of the above heat treatment is not dependent directly on the component composition of the hot rolled steel sheet, so the heat rolled steel sheet of the present invention is basically not limited to the hot rolled steel sheet of a specific component composition. It is preferable that C: 0.07 to 0.50%, Si: 0.10 to 1.00%, and Mn: 0.70 to 3.00%.

  C is an element necessary for securing the strength of the molded product, and in the heat-rolled steel sheet of the present invention, mass% (hereinafter simply referred to as “%”) to secure 980 MPa or more as the strength after hot stamping. And preferably 0.07% or more. More preferably, it is 0.10% or more. On the other hand, if it exceeds 0.50%, it becomes too hard and the cold workability decreases, so the content is made 0.50% or less. Preferably it is 0.40% or less.

  Since Si is an element contributing to the improvement of strength, 0.10% or more is preferable. More preferably, it is 0.30% or more. On the other hand, if it exceeds 1.00%, it becomes too hard and the cold workability decreases, so 1.00% or less is preferable. More preferably, it is 0.50% or less.

  Mn is an element that enhances the hardenability and contributes to the improvement of the strength, so 0.70% or more is preferable. More preferably, it is 1.00% or more. On the other hand, if it exceeds 3.00%, it becomes too hard and the cold workability decreases, so 3.00% or less is preferable. More preferably, it is 2.70% or less.

  In the present invention, in addition to C, Si and Mn, P: 0.010% or less, S: 0.010% or less, Al: 0.001 to 0.100%, N: 0.010% or less And O: 0.010% or less may be contained.

  Since P and S are impurity elements, they are preferably as small as possible, and both are preferably 0.010% or less. More preferably, all are 0.005% or less.

  Al is a deoxidizing element, and preferably 0.001% or more to obtain a deoxidizing effect. More preferably, it is 0.005% or more. On the other hand, when it exceeds 0.100%, coarse oxides are formed to inhibit the cold workability of the hot rolled steel sheet, so 0.100% or less is preferable. More preferably, it is 0.050% or less.

  N is preferably 0.010% or less because it is an element which is inevitably mixed from an iron raw material. More preferably, it is 0.005% or less.

  Since O is an element which remains unavoidably after deoxidation, 0.010% or less is preferable. More preferably, it is 0.005% or less.

  The heat-rolled steel sheet of the present invention may appropriately contain Ca, REM, Mo, Nb, V, Ti, Zr, Cr, Ni, B, Cu, etc. in addition to the above elements, as long as the characteristics of the heat-rolled steel sheet of the present invention are not impaired. May contain an amount of

  In the heat-rolled steel sheet of the present invention, the balance other than the elements constituting the component composition is Fe and unavoidable impurities.

  Next, although the Example of this invention is described, the conditions in an Example are one condition example employ | adopted in order to confirm the practicability and effect of this invention, and this invention is the one condition example. It is not limited. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the scope of the present invention.

Example 1
A slab of the composition shown in Table 1 is produced by continuous casting, and hot rolling is performed under the conditions shown in Table 2 and Table 3 (continuation of Table 2) to obtain a hot rolled steel sheet, and cooling under the conditions shown in the table And it reheated and wound up as it was. That is, after cooling to 300 ° C. or less in the cooling zone, it was heated to 400 ° C. or more and 750 ° C. or less, taken up as it was, and the hardness Hv (load: 1 kg) was measured with a Vickers hardness tester.

  The tissue was observed using SEM and EBSD to calculate the ferrite fraction. The cold workability of the heat-rolled steel sheet of the present invention was evaluated by Vickers hardness. Vickers hardness: An Hv of 350 or less was regarded as an excellent range of cold workability.

  Furthermore, each hot-rolled steel plate is cold-rolled to a thickness of 1.4 mm and cut into a size of 100 mm × 200 mm, and then the heat history of the hot stamping process is simulated to load in an electric furnace Thereafter, a holding heat treatment was performed at 900 ° C. for 3 minutes, and then hardening was performed with a die using a press. The Vickers hardness was measured to evaluate the strength of the steel sheet after this quenching. The obtained results are shown in the same table.

  In the heat-rolled steel plate according to the invention method, a value of 350 Hv or less is obtained in Vickers hardness, and it is understood that the cold workability is secured. Moreover, the hardness corresponding to the amount of C is also obtained as the hardness after hot stamping. However, since the hardness of the hot rolled steel sheet subjected to the hot rolling in which the finishing temperature exceeds 3 Ar points exceeds 350 Hv, the cold rolling is difficult.

  When the cooling end point temperature deviates high from the range (300 ° C. or less) of the present invention, or when the reheating temperature and the winding temperature deviate high from the range (400 ° C. to 700 ° C.) according to the present invention The carbides formed in the stage become coarse, and do not form a solid solution sufficiently at the time of heating at the time of hot stamping, and the hardness after martensitic transformation is insufficient.

  In the case of J steel in which C deviates low from the range of the present invention (0.07 to 0.50%), sufficient hardness is not obtained, but in K steel in which C deviates high from the range of the invention, The hardness is too high. In L steel where Si deviates high from the range of the present invention (0.10 to 1.00%), sufficient strength is not obtained due to the promotion of ferrite transformation occurring during cooling and the formation of retained austenite .

  In addition, in the case of M steel in which Mn deviates high from the range of the present invention (0.70 to 3.00%), the formation of retained austenite also does not provide sufficient strength. On the other hand, when cooling after finish rolling is completed at a temperature higher than 300 ° C., and if the reheating temperature exceeds the range of the present invention, carbides precipitated in the hot-rolled steel sheet are coarsened and the hot stamp is obtained. In the heating step at the time of implementation, the solid solution did not fully resolute, and the hardness corresponding to the amount of added C was not obtained.

  Also, as in the conventional case, when the finishing temperature is higher than the Ar3 point, the hardness of the hot rolled steel sheet becomes too high and the load at the time of cold rolling is excessively increased because the ferrite fraction of the hot rolled steel sheet is low. Are concerned.

(Example 2)
Using a steel D shown in Table 1, a hot-rolled steel plate having a thickness of 2 mm was manufactured, and thereafter, the hot-rolled steel plate was cooled under cooling conditions within the scope of the present invention, and cooled as it was to room temperature. Subsequently, the wound hot rolled steel sheet was rewound and subjected to reheating treatment, and when rewound, reheating treatment was performed with the heat history shown in Table 4 .

About the obtained hot-rolled steel plate, hardness Hv (load: 1 kg) was measured with a Vickers hardness tester like Example 1. The results are shown together in Table 4 .

  In the heat-rolled steel plate according to the invention method, ferrite is sufficiently secured, and a hardness of 350 Hv or less is obtained.

Also, simulating the hot stamping process, the heat-rolled steel plate (test piece) was heated to 900 ° C. at 100 ° C./sec by electric heating, held for 3 seconds, and immediately cooled to room temperature. After cooling, the hardness of the test piece was measured with a Vickers hardness tester. The results are shown together in Table 4 .

  In the heat-rolled steel plate according to the invention method, a hardness of 350 Hv or less is obtained, and good cold workability is ensured. Moreover, as for the hardness of the hot rolled steel sheet after hot stamping, the hardness corresponding to the amount of C is obtained.

  According to the invention method, in the heat-rolled steel plate, since ferrite is sufficiently secured, a hardness of 350 Hv or less is obtained, and cold workability is secured. Moreover, as for the hardness of the hot rolled steel sheet after hot stamping, the hardness corresponding to the amount of C is obtained. On the other hand, when the temperature of the reheat treatment exceeds the range of the present invention, the hardness after hot stamping is significantly reduced.

  As described above, according to the present invention, the microstructure of the hot-rolled steel sheet is uniformed and the cold workability is improved. Therefore, the reduction ratio is increased to manufacture a thin cold-rolled steel sheet having a uniform microstructure. Is possible. As a result, the dimensional accuracy of the hot stamped molded product is improved, and variations in hardness and strength are eliminated. Therefore, the present invention is highly applicable in the steel sheet manufacturing industry.

S 'rough bar S, S1 hot rolled steel sheet

Claims (2)

C: 0.07 to 0.50%, Si: 0.10 to 1.00%, Mn: 0.70 to 3.00%, P: 0.010% or less in mass% in a continuous hot rolling process S: 0.010% or less, Al : 0.001 to 0.100 %, N: 0.010% or less, O: 0.010% or less, and further, by mass%, Ti: 0 to 0.. B: 0 to 0.0011%, Cr: 0 to 0.15%, and Mo: 0 to 0.12% of one or more kinds, the balance being Fe and an unavoidable impurity, have a microstructure containing 20% or more of ferrite volume fraction, a method of Vickers hardness to produce a superior hot-rolled steel sheet cold-rolling property is less than 350 Hv, the temperature below 600 ° C. or higher Ar3 point The hot-rolled steel plate that has finished hot rolling in the temperature range is cooled to 300 ° C or less, and it is 400 ° C or more and 700 ° C just before winding up. Heating below, a manufacturing method of hot-rolled steel sheet, characterized in that for winding as it is. C: 0.07 to 0.50%, Si: 0.10 to 1.00%, Mn: 0.70 to 3.00%, P: 0.010% or less in mass% in a continuous hot rolling process S: 0.010% or less, Al : 0.001 to 0.100 %, N: 0.010% or less, O: 0.010% or less, and further, by mass%, Ti: 0 to 0.. Ferrite containing one or more of 011%, B: 0 to 0.0011%, and Cr: 0 to 0.15%, the balance being Fe and unavoidable impurities, and having a volume fraction of 20% or more It has a microstructure containing a Vickers hardness of a method for producing a good hot rolled steel sheet cold-rolling property is not more than 350 Hv, to complete the hot rolling at a temperature range of not lower than 600 ° C. or less Ar3 point heat The rolled steel sheet is cooled to 300 ° C. or less and wound up, and then rewound and added to 400 ° C. to 700 ° C. Method for manufacturing a hot-rolled steel sheet, characterized in that to winding.

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